In applications such as high pressure chromatography, it is necessary to mix two fluids, which fluids may be either liquids or gases, each of which is received from a high pressure source. Since the pressures under which fluids are delivered in such a system may be high, 6,000 psi. or higher in a super critical fluid chromatography (SFC) system, variations in pressure from the two sources can result in a number of potential problems.
A first problem is that it is necessary to isolate fluid being pumped from one source from fluid being pumped from the other source under situations where fluid is being delivered only from one of the sources, where both pumps are turned off, but where normal pressure from one source is greater than than that from the other, or in other situations where there is greater pressure from one source than from the other. Without such isolation, fluid from one source may contaminate the other source as a result of back flow into its supply line through the mixer, and it may be necessary to purge the line leading from the turned off or low pressure source or to prime its pump, both of which take time and reduce the operating efficiency of the system. Contamination, if not purged, can also result in erroneous measurements being taken, or other problems in the system.
It is also desirable that the flow of fluids into the mixer be substantially uniform regardless of slight flow fluctuations from the pumps. It is, therefore, desirable that such fluctuations be buffered to permit uniform fluid flow.
Systems do not currently exist for dealing with the problems indicated above in a high pressure fluid mixing environment.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide an improved valve assembly for controlling pressurized inputs from two separate fluid sources which provides enhanced isolation between the sources, provides backflow protection, and which buffers short-term fluctuations in fluid flow between the fluid sources.
More particularly, this invention provides a valve assembly of the type indicated above which has a chamber with first and second opposed sides. Each of the sides has an inlet port and an outlet port with an input from a different one of the fluid sources be connected to each of the inlet ports. A valve having a movable member such as a ball is mounted in each inlet port to control flow therethrough. The movable member is movable between a position where it closes the valve to block flow through the port and positions successively removed from the port where progressively greater flow through the port is permitted. A flexible membrane is mounted in the chamber between the sides to separate the chamber into two substantially isolated chamber portions. The membrane is flexed when there is a pressure differential in the chamber portions to bear against the movable member for the valve of the lower pressure chamber portion to selectively control flow through this valve. The outlet ports are connected through separate restrictors to a mixer and the valve assembly may be utilized in a high pressure SFC system, with the output from the mixer being applied to an analysis column of such system. The restrictors create a pressure in the chamber portions which vary with flow. Thus, when there is a difference in flow from the sources, the restrictor results in a pressure difference between the two chambers which is sufficient to move the membrane but is small enough that the membrane does not burst.
This pressure difference should range between a few millibars to approximately 1 bar, depending on factors such as viscosity and flow of the respective fluids. The membrane is preferably formed of a material which is substantially impervious to the fluid.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.